The present invention relates to an active-matrix type liquid crystal display device using thin film transistors and to a manufacturing method therefor.
In recent years, active matrix display has been forming the mainstream of the display method for LCD (Liquid Crystal Display) devices. An LCD device of this active matrix type consists of two substrates and liquid crystals sandwiched between the two substrates, where switching devices formed in correspondence to pixels arrayed in a matrix shape as well as pixel electrodes connected to the switching devices are formed on one substrate, while an opposite electrode is formed on the other substrate. In the LCD device of this constitution, a display is effected by applying a display signal to between the pixel electrodes and the opposite electrode. More concretely, by applying a display voltage, the state of orientation of liquid crystals sandwiched between the two electrodes is changed, so that the quantity of light that is transmitted by these liquid crystals is controlled, by which a display is achieved.
As the switching devices, nonlinear devices such as thin film transistors (hereinafter, referred to as TFTs), diodes and the like are used. Among others, polysilicon TFTs that can be formed integrally with drive circuits of the LCD device and that are fast in response speed are commonly used. However, in the active matrix LCD device, even if light that impinges directly on the TFT is intercepted by limiting the direction of incidence of light, light that has passed through portions where no light interception film is formed may, in some cases, return to the TFT side as a result of reflection by optical part formed outside the LCD device (lens, polarizer, mirror etc.) or by the inner wall of the LCD device or the like. Particularly in the case of an LCD device used for projection type display devices, since very intense light is irradiated for enlarged projection of images, a large amount of light returns to the TFT side (return light), causing a considerable characteristic deterioration of TFTs due to this return light, as a problem.
Under these circumstances, to solve these and other problems, there has been proposed a structure in which a light interception film is formed on the TFT substrate side where a polysilicon TFT is formed, or over a region on the opposite substrate side corresponding to the site where the TFT is formed (e.g., Japanese Patent Laid-Open Publication HEI 6-138483). As one of such LCD devices, there has been provided an LCD device in which a polysilicon single layer is formed as a light interception film on the transparent substrate, the film thickness of the polysilicon single layer being set so that the return light is intercepted by the interference effect. However, in the LCD device, because of insufficient light interception of the polysilicon single layer itself as well as a difficulty of film thickness control in obtaining necessary light interception performance, there is a problem that enough light interception effect could not be obtained.
Thus, there has been provided another LCD device, as shown in FIG. 5, in which a metal thin film is formed as a light interception film on a transparent substrate. This LCD device, as shown in FIG. 5, has a tantalum film 52 formed on a transparent substrate 51, a transparent insulation film 54 formed on the transparent substrate 51 and on the tantalum film 52, a TFT-use polysilicon layer 55 formed over a region on the transparent insulation film 54 corresponding to the tantalum film 52, a gate insulation film 56 formed on the transparent insulation film 54 and on the TFT-use polysilicon layer 55, a gate electrode 57 formed over a region on the gate insulation film 56 corresponding to a generally central portion of the TFT-use polysilicon layer 55, a first interlayer insulation film 58 formed on the gate insulation film 56 and on the gate electrode 57, TFT-use metal interconnections 59, 60 formed on both sides of the TFT-use polysilicon layer 55 and on the first interlayer insulation film 58, a second interlayer insulation film 61 formed on the first interlayer insulation film 58 and on the TFT-use metal interconnections 59, 60, and a pixel electrode 62 formed on the second interlayer insulation film 61 and partly connected to the TFT-use metal interconnection 60. By the formation of the metal thin film (tantalum film 52) as a light interception thin film on the transparent substrate 51, characteristic deterioration of TFTs due to the light that returns to the TFTs (return light) is prevented.
However, in the LCD device shown in FIG. 5, since only a metal thin film (tantalum film 52) is used as a light interception layer, poor adhesion is obtained between the metal thin film 52 and the transparent insulation film 54 formed on top of the metal thin film 52. As a result, there are problems that film peeling occurs due to high-temperature heat treatment in later TFT manufacturing process, and that light incident between the TFT and the metal thin film 52 is reflected by the metal thin film 52 so as to be incident on the TFT.
Therefore, an object of the present invention is to provide a liquid crystal display device, as well as a manufacturing method therefor, capable of preventing characteristic deterioration of TFTs by reducing the amount of return light incident on the TFTs, and of preventing film peeling even with high process temperatures.
In order to achieve the aforementioned object, in one aspect of the present invention, there is provided a liquid crystal display device comprising: a transparent substrate; a transparent insulation film formed on the transparent substrate; a semiconductor thin film transistor formed on the transparent insulation film; and a light interception film formed over a region corresponding to the semiconductor thin film transistor between the transparent substrate and the transparent insulation film, wherein
the light interception film has a first thin film composed of silicide formed on or above the transparent substrate, and a second thin film composed of semiconductor formed so as to cover at least a top of the first thin film.
In the LCD device of this constitution, since the light interception film includes the first thin film made of silicide and the second thin film made of semiconductor deposited on at least top of the first thin film, light that returns to the TFT side (return light) can be intercepted more effectively, as compared with the light interception effect by a polysilicon single layer. Also, in comparison with a light interception film formed of a metal single layer, the adhesion between the first thin film made of silicide and the transparent substrate is improved, while the adhesion between the second thin film made of semiconductor and the transparent insulation film formed on the top of the second thin film is improved. A good adhesion is also obtained between the first thin film made of silicide and the second thin film made of semiconductor deposited on at least the top of the first thin film. Further, since the second thin film made of the semiconductor is formed on at least the top of the first thin film made of silicide, the return light that has entered between the TFT and the light interception film can be absorbed by the second thin film made of the semiconductor, so that reflection is suppressed. Thus, by effectively intercepting the return light incident on TFTs, an LCD device capable of preventing characteristic deterioration of the TFTs and moreover preventing film peeling even at high process temperatures can be realized.
In one embodiment of the invention, the first thin film is made of a high melting point metal silicide.
According to the LCD device of this embodiment, by using silicide having a high melting point metal material as the first thin film, the thermal resistance of the light interception film is improved, and in particular, a TFT can be produced at high process temperatures in the heat treatment of the TFT manufacturing process. Thus, a TFT of good characteristics can be obtained.
In one embodiment of the invention, the second thin film is made of a Si-series semiconductor or a Ge-series semiconductor.
According to the LCD device of this embodiment, by using Si-based semiconductor or Ge-based semiconductor as the semiconductor material of the second thin film, the adhesion of the second thin film with the transparent insulation film, such as silicon oxide or silicon nitride, formed on at least the top of the second thin film is further improved.
In one embodiment of the invention, the light interception film further comprises a third thin film made of semiconductor formed between the transparent substrate and the first thin film made of silicide.
According to the LCD device of this embodiment, by forming the third thin film made of semiconductor between the transparent substrate and the first thin film made of silicide, the adhesion between the light interception film and the transparent substrate is improved.
In one embodiment of the invention, the second thin film made of semiconductor covers a side portion of the first thin film made of silicide.
According to the LCD device of this embodiment, since the semiconductor thin film is formed also at a side portion of the first thin film made of silicide, impurities can be prevented from being mingling into the TFT-use polysilicon layer side due to thermal diffusion in the TFT manufacturing process. Thus, characteristic deterioration of TFTs due to adulterant impurities can be prevented.
In one embodiment, the semiconductor thin film transistor is a polysilicon thin film transistor.
In one embodiment, the transparent insulation film is a silicon oxide film or a silicon nitride film.
In one aspect of the invention, there is provided a method for manufacturing a liquid crystal display device comprising the steps of:
forming a first thin film made of silicide on or above a region on a transparent substrate above which a semiconductor layer of a semiconductor thin film transistor is to be formed;
forming a second thin film made of semiconductor on at least a top of the first thin film made of silicide;
forming a transparent insulation film on the transparent substrate and on the second thin film made of semiconductor; and
forming the semiconductor layer of the semiconductor thin film transistor on a region on the transparent insulation film corresponding to the first thin film made of silicide.
According to the LCD device manufacturing method of this constitution, since the melting point of the silicon film is about 1400xc2x0 C. and the melting point of the silicide film is 1300-1500xc2x0 C. or higher, enough thermal resistance and light interception can be obtained not only when ordinary backlight is used but also when lamps that emit intense light such as projection-use halide lamps are used. Also, by using such a light interception structure, it becomes possible to implement a high-temperature heat treatment with the process highest temperature falling within a range of 900-1200xc2x0 C. without causing any film peeling between the light interception film and the transparent insulation film or any characteristic deterioration of TFTs due to thermal diffusion. Thus, the TFT manufacturing processes can be carried out at high process temperatures, and a TFT of good characteristics can be obtained.
In one embodiment, the semiconductor thin film transistor is a polysilicon thin film transistor and the semiconductor layer is a polysilicon layer.
One embodiment further comprises a step for forming a third thin film made of semiconductor on the transparent substrate before forming the first thin film, the third thin film being placed between the transparent substrate and the first thin film.
In one embodiment, the third thin film is made of polysilicon.